Electrical resistance element and method of making the same



y 196 A M. DAILY AL 3,329,526

LEMENT AND ELECTRICAL RESISTANC METHOD OF MAKING THE SAME Original Filed Aug. 15, 1961 RESISTANCE FILM LESS THAN .0005" THICK COMPRISING A NO METAL IN A ECULAR STATE OF SUB-DIVISION PERSED IN GL FIRED ONTO BASE INVENTORS ARTHUR M. DAILY OTIS F. BOYKIN CL I NTON W. HAR'TMAN ATTO NEY United States Patent 20 Claims. (Cl. 117227) The present application is a continuation of our copending application entitled Electrical Resistance Element and Method of Making the Same, Ser. No. 131,491 filed Aug. 15, 1961, now abandoned which application is in turn a continuation-in-part of our application entitled Metal Film Resistor and Method of Making the Same, Ser. No. 386,922, filed Aug. 31, 1959, now abandoned. The entire disclosures of both of the above applications are incorporated in the present application by reference.

This invention relates to the art of making electrical resistance elements generally and refers more particularly to resistance elements of the type in which the resistance path consists of minute crystals of metal dispersed in a thin film of glass fired onto a base or substrate of high temperature-resistant insulating material, such as ceramic.

- More specifically, the object of the instant invention is the attainment of a ceramic-metal resistance element that can be consistently produced with the resistance values thereof lying within prescribed tolerance limits in the higher resistance ranges.

Certain significant and important discoveries have made it possible for the first time to produce ceramic-metal resistance elements which not only retain all of the previously found advantages of this type resistance element, such as the ability to perform satisfactorily and reliably under the conditions prevalent in guided missiles and rockets; but which also can be consistently produced with overall resistances held (within close tolerance limits) to any predetermined value in preferably the high resistance range of from 50,000 to 400,000 ohms per square, as well as in the lower ranges.

The significance of this new development in this art is readily apparent when it is remembered that variable resistance controls have in recent years become physically smaller and smaller. This is particularly true of the variable resistance controls of both the rotary type and the rectilinear type employed in the complex electronic control systems of guided missiles and rockets. The controls range in size from one-half inch to one and one-half inches in diameter for the rotary type and from three-fourths of an inch to one and one-fourth inches in length for the rectilinear type. The limits on the physical size of each of the controls also determine the maximum physical size of the resistance element used in the control. This in turn effectively determines the maximum overall ohmic resistance available for any given size control since the overall ohmic resistance of a resistance element is calculated in advance by solving for the number of squares in the element and multiplying this number by the resistance per square of the particular composition to be used in the element. The number of squares in any given element is determined by dividing its mean length by its width. For any given control, the mean length is fixed for all practical purposes so the only variables left are the thickness, the width and the resistance per square of the composition used in the element. In other words, to produce changes in the overall resistance of the element, either its thickness, width, or resistance per square must be altered.

The width of the element is subject to only limited adjustment. It must be wide enough to handle the power requirements of the control and it must have suflicient I flow of electricity. Consequently such other noble metals,

width to have good electrical contact with the contact button which engages it. If it is made too narrow, the contact resistance will be too high and/or the power capacity of the control will be too limited. For these reasons, the practical limits to the ratio of width to length for any control between one-half inch and one and onehalf inches in diameter or between three-fourths of an inch and one and one-fourth inches long produces an element having a maximum of approximately twenty squares. With this limit on the number of squares available, to produce variable resistance controls having overall resistances ranging from 5,000 ohms to 8 megohms, the resistance per square must range from 250 ohms per square to 400,000 ohms per square.

From the exhaustive tests we have conducted, it is now known that resistance elements of this type can be consistently produced by applying the film in one or more layers applied one at a time until the desired resistance value is obtained.

Of the noble metals, ruthenium possesses the unique property of producing resistance elements having the highest resistance values. The answer may lie in the fact that ruthenium has a unique crystalline structure. Ruthenium has a hexagonal close-packed crystalline structure which has an acicular growth. All other noble metals with the exception of osmium have a face-centered cubic crystalline structure, which grows uniformly along all its faces, thereby producing a granular structure. Osmium is not available in the form of an organic metal compound such as a metal resinate, and hence is apparently not usable in the practice of this invention but the other noble metals such as platinum, palladium, silver, gold and iridium, disclosed in our application Ser. No. 836,922, now abandoned, can be employed in making resistance elements.

For the ceramic-metal resistance elements to be conductive at all, there must be a continuous conductive path from one end thereof to the other. The long needle-like crystals of ruthenium can produce a continuous conductive path through the film which will have a high length to cross-sectional area ratio, and hence will have very high resistance. The granular crystalline structure of the other noble metals cannot do this. A continuous conductor path of the crystals of these other noble metals would, of necessity, have low length to cross-sectional area ratios, and would, therefore, offer considerably less resistance to the are preferable for producing resistance elements having a resistance value below that of those produced with ruthenium.

This is believed to be the reason why ruthenium alone can reliably produce ceramic-metal resistance elements in the 50,000 to 400,000 ohms per square resistance range.

attainment of the objectives of this invention is the control of the film thickness, and this is true whether the metal content of the film is ruthenium or another of the other noble metals such as platinum; palladium and silver; platinum and gold; platinum and iridium; and palladium, iridium and gold, all of which are disclosed in our parent application, Ser.'No. 836,922. Heretofore, ceramic-metal films of this type have been on the order of .0005 to .003 inch thick. This is the range of film thickness which Place et al. state is required in their Patent No. 2,950,995. Their theory is that the film should be relatively thick, and that the resistivity of the film should be controlled by the composition of the film. We have found, however, that the total film thickness must be Well below .0005 inch in thickness, especially if the desired high resistance values are to be obtained.

The film is produced by firing a mixture containing ground glass frit, ruthenium organosol, or the organosol of some other noble metal, depending upon the range of resistance contemplated, and an organic screening and viscosifying agent onto a ceramic base. It is essential, though, that the film be formed in successive separately fired layers, no one of which has a thickness greater than .0000-5 of an inch.

The thickness of the complete film usually averages .00009 inch, and seldom if ever exceeds .00021 inch. These thicknesses are respectively those of a film produced by successively firing three and seven separate layers, respectively, each layer averaging around .00003 of an inch in thickness when fired.

With such extremely thin films, the size of the ruthenium or other noble metal particles dispersed therein must, of necessity, be extremely small. This fact was substan tiated by a series of electron photomicrographs taken of a group of the fired films made with ruthenium organosol. These photomicrographs showed that the ruthenium crystals in the film were all less than two microns in size, with the majority being less than one micron in size.

While the thickness of the successively fired layers of the complete film has not been actually measured, it can be calculated by simply multiplying the thickness of the applied mixture before firing by the percentage of glass in the mixture. This affords a relatively accurate way of determining the thickness of the fired film since, as will be more fully explained hereinafter, the screening and viscosifying agent constitutes by far the major part of the mixture, usually being in excess of 85% by weight thereof; and the relative proportions of glass and ruthenium or other noble metal organosol present in the mixture are such that after firing, the amount of ruthenium or other noble metal dispersed in the glass is on the order of 2% by weight of the whole. Thus, it is primarily the percentage of ground glass in the unfired mixture which determines the thickness of the fired film. The percentage of metal may be ignored since, in most cases, it would have a negligible effect on the thickness of the film, and all the other components of the mixture are driven or burnt off during the firing. Any slight error made in the measurement of the thickness of the applied mixture will thus have little effect upon the accuracy of the calculated thickness of the fired film.

In the silk screening process, which is the preferred way of applying the mixture, the screened layers are generally between .001 and .003 inch thick. The percentage of glass present in the mixture, therefore, must be such that with the thickness of the screened layer before firing lying between these limits, the thickness of the fired film will not exceed .00005 of an inch. This presents no difficulties. For example, if the thickness of the screened-on layer before firing is .002 inch, 2% ground glass (by volume) in the mixture will result in a fired film thickness of .00004 inch which is well within the critical limit of .00005 inch for each individual layer.

There are two important reasons Why the thickness of each fired layer should be held to less than .00005 inch. The first of these concerns the obtention of predictable resistance values; the second has to do with the firing rate.

Obviously it would be advantageous from every standpoint and particularly economy of production if the desired resistance element could be obtained with one firing. All efforts to do this have been unsatisfactory to date. This is particularly true in the high resistance ranges. Thus, for instance, in the 250,000 to 300,000 ohms per square range, we have been able to consistently produce resistance elements with their resistance values lying between prescribed tolerance limits only after firing from two to four layers of a mixture which produces a film containing 2% ruthenium by weight.

If the percentage of glass and the percentage of ruthenium organosol in the mixture is simply increased so that a film equal in thickness and containing the same ratio of metal to glass as that of the multiple layer element is produced in one firing, the result is not the same.

The resistance may be over or under the desired value, and while a small percentage of the elements may even lie within the prescribed tolerance limits, the results are definitely not predictable. Only by applying layer upon layer and keeping each fired film less than .00005 inch in thickness can resistance elements be consistently and repeatedly produced with their resistance values lying within prescribed tolerance limits.

The extremely thin film thickness of each of the successively applied and fired films moreover allows a fast firing cycle. These very thin films can be fired in less than thirty minutes. This 'would not be possible with a thicker lm because of the tendency of the glass to blow. Blowing results when a mixture having a high percentage of glass is fired rapidly. The glass on the top softens and melts, tending to seal the top of the film before all of the organic compounds are volatilized. Gases are thus trapped below the surface and as they build up in pressure they eventually overcome the surface tension of the molten glass and erupt, leaving holes and other surface irregularities which are very undesirable. To prevent this, mixtures containing more than 4-5% glass (by volume, in the unfired state) when screened to .001 inch or thicker, must be fired very slowly to allow for the escape of most of the gases before the melting point of the glass is reached. On the other hand, with the extremely thin film of this invention, the melting glass has insufficient thickness to trap the gases produced during the firing. Hence the firing can be done rapidly without danger of any blowing of the glass matrix.

Another advantage of having the film as thin as it is is that it permits the use of a glass which has a different temperature coefficient of expansion than that of the ceramic base. Keeping the film as thin as it is in this invention avoids the usual problems of crazing and checking which result when the glass and the ceramic base upon which it is fired have different temperature coefficients of expansion.

As noted hereinbefore, the thickness of the fired film can be calculated by multiplying the thickness of the unfired layer by the percentage of glass in the mixture. As to the very first layer, this is not entirely correct due to the phenomena known as feeding of a glaze on the ceramic base. The glass used to produce the glass phase of the film is preferably a lead borosilicate glass. When such glass is fired at 8008S0= C., it becomes very corrosive and will attack the ceramic base. Therefore, the thickness of the film after firing will be slightly less than that calculated, the difference being in the amount of glass lost to the ceramic base. However, the volume of glass lost in this manner is not great and the resultant strong bond created between the ceramic base and the glass film is very desirable.

Before applying the mixture to the surface of the ceramic base to produce the initial layer of the film, the ocramic surface of the ceramic base should be lapped and polished to make it as smooth as possible. The smoother this surface is, the better the chances are of attaining the desired consistency in production. It also produces an element with a smoother surface, which is an extremely important matter in resistance elements for variable resistance controls.

Although lapping and polishing of the ceramic base is done as carefully as possible, some pits and cavities inevitably remain and, as a result, the first layer of the film fired onto the base often has an infinite resistance. This is believed to result from the molten glass of this first layer collecting in the pits and cavities on the surface of the base and, in so doing, disrupting the continuity of the conductor path. However, the subsequently applied and fired layers consistently produce predetermined resistance values.

A grossly enlarged cross section of a resistance element in the single figure of the drawing.

Percent Percent by Weight by Volume Ground glass irit 8. 70 1. 68 Ruthenium organosol (4% ruthenium by weight) 4. 35 4. 46 Screening and vlscoslfying agent 86. 95 93. 86

The above mixture when fired will produce an element containing approximately 2% ruthenium. Before firing, the ruthenium comprises 174% by weight and .015 by volume of the mixture. This low percentage mixture is used in this case since the physical dimensions of the base limit the number of squares to 13, thus requiring an element having approximately 385,000 ohms per square.

The composition of the glassused is not critical, nor is the manner in which it is produced. As an example, the glass frit may be obtained by melting together boric oxide, silicon dioxide, and lead oxide and pouring the molten mixture into cold water. This produces a glass frit which is then ground to a fine particle size, preferably less than 325 mesh.

The ruthenium organosol may be any one of those commercially available, and preferably the actual metal present therein is 4% by weight. This is not critical, however, as long as the ratio of ruthenium to glass is controlled since all of the organosol except the ruthenium will be driven off during the firing operation. The same, of course, holds true if some other noble metal organosol is used. When the term organosol is used herein it is intended to mean, as it did in our aforesaid parent application, organic metal compound in solution, and as also stated in our parent application, the organic metal compounds that are here contemplated are the metal resinates, glycinates, etherates, esterates, and naphthanates. Their solvents usually are one of the mineral or vegetable oils.

Any of the well known screening agents which are capable of being completely volatilized or decomposed by heat can be used. A viscosifying agent should be present in the mixture also, to help keep the ground glass frit in suspension after the mixture has been screened on the base. Ethylcellulose dissolved in a trichloroethylenefenchone solution is an example of a screening agent which also serves as a viscosifying material.

The mixture is prepared as follows:

(1) The ground glass frit and the selected organosol are mixed together in the desired proportion.

(2) This liquid mixture is then ground to effect thorough dispersion of the glass powder in the metallic organosol.

(3) The viscosifying and screening agents are then added and the resulting admixture is ready for application to the ceramic base or substrate.

A layer .002 inch thick of the mixture is then screened in the desired pattern onto the ceramic base and fired at 150 C. for ten minutes, 350 C. for ten minutes and 800 C. for five minutes. At the 150 C. level, most volatile components are driven from the mixture. At the 350 C. level, the reduction of the organosol begins as does the burning of the carbonaceous residue. The reduction of the organosol probably is not complete until the 800 C. temperature level is reached. At this point only the glass in the molten state and the ruthenium or other noble metal produced by the reduction of the organosol remain on the substrate. The metal comes out of the resinate as it is reduced and forms minute crystals of the metal. As disis proven by the fact that each square of the film exhibit uniform electrical properties.

The base with the fired-on film is removed from the kiln in which the firing took place and cooled in air. The resistance of the element is then measured so that an estimate can be made of the number of additional layers which will be needed to reach the desired resistance value. As explained above, this initial resistance value will depend, among other things, upon the surface condition of the base.

In the example being considered, the 5 megohm total resistance desired was obtained in 36 out of the 50 units that were prepared after three layers had beeen fired, while of the remaining, 12 required an additional firing, or four layers, to reach 5 megohms of total resistance. The other two elements were discarded because they were not within tolerance after the fourth firing.

The finished resistance film of these elements consisted of slightly in excess of 98% glass and slightly less than 2% ruthenium dispersed throughout the glass film. The calculated thickness of the finished resistance film for the 36 units which had only three layers was .00009 inch, while for those units which required an additional firing, it was .00012 inch.

When lower resistance values are desired, the mixture is altered to increase the precentage of ruthenium in the film, or an organic metal compound of another noble metal can be used. A maximum of about 13% is believed to be the practical limit in the percentage of ruthenium which can be used with success. Films with higher percentages are unpredictable and exhibit undesirable electrical characteristics. In all cases, however, the film must be maintained under .0005 inch in thickness to produce satisfactory results. The percentage of ruthenium present in the complete film should lie within the range of /2 to 13% by weight, and, of course, the balance of this film is glass.

From the foregoing description, it will be apparent to those skilled in this art that this invention provides an electrical resistance element and method of making the same which possesses many important advantages over those heretofore available. They will also recognize that the specific embodiment of the invention hereinbefore described is but an example of the best mode in which the principles of the invention have thus far been embodied, and that such changes may be made in the described pro cedure, and in the product thereof as come within the appended claims, without defeating the objectives of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electrical resistance element of the type comprising a film of glass on a base of high temperature resistant insulating material containing minute crystals of metal dispersed therein, characterized by the fact that said metal is ruthenium, the ruthenium containing glass film being less than .0002 inch thick, .said ruthenium having a hexagonal close-packed crystaline structure and an acicular growth.

2. An electrical resistance element having a resistance per square lying in the range of 50,000400,000 ohms and comprising a high temperature resistant non-conductive base, and an electrically resistive film of less than .0002 inch in thickness fired thereon and comprising 8799.5% by weight glass and from 05-13% by weight ruthenium uniformly dispersed throughout the glass.

3. A composition adapted to be fired onto a ceramic base to form a consistently reproducible resistance path comprising by weight 4*9% finely ground glass frit; .04-

.64% ruthenium, said ruthenium being present as an organic metal compound in solution; and 91-96% organic screening agent.

4. The method of manufacturing an electrical resistance element having a resistance lying in the range of 50,000- 400,000 ohms per square and having a thickness less than .0002 inch comprising:

(a) depositing a mixture comprising by volume 0.5-

2.0% glass frit, .003-.06% ruthenium, said ruthenium being present in the mixture as an organic metal compound in solution, and 98-99.5% organic screening agent on a ceramic base in a layer from .001-.003 inch thick;

(b) firing the base and the mixture deposited thereon to a tempreature sufficient to melt the glass frit, reduce the ruthenium organic metal compound in solution, and volatilize the organic screening agent and burn off the carbonaceous residue, but below the temperature necessary to soften the ceramic base, to produce a film of glass on the base containing by weight -13% ruthenium;

(c) cooling the base and film in air to solidify the glass;

(d) measuring the electrical resistance of the film;

and

(e) repeating steps (b) and (0) until the desired resistance value is obtained.

5. The method of manufacturing an electrical resistance element as set out in claim 4 further characterized by the fact that the total firing time for step (b) is less than one hour.

6. The method according to claim 4 in which step (b) comprises:

(1) heating the base and the mixture deposited thereon to approximately 150 C. for from five to fifteen minutes, and

(2) firing the base and the mixture deposited thereon at approximately 800 C. for from two to ten minutes.

7. The method according to claim 4 in which step (1)) comprises:

(1) heating the base and the mixture deposited thereon to approximately 150 C. for from five to fifteen minutes,

(2) heating the base and the mixture deposited thereon to approximately 350 C. for from five to fifteen minutes, and

(3) firing the base and the mixture deposited thereon at approximately 800 C. for from two to ten minutes.

8. The method of manufacturing an electrical resistance element having a resistance lying in the range of 50,000-400,000- ohms per square, comprising:

(a) depositing a mixture comprising by volume 0.5-

2.0% glass frit, .0O3-.06% ruthenium, said ruthenium being present in the mixture as an organic metal compound in solution, and 98-99.5% organic screening agent on a ceramic base in a layer from .00l-.00-3 inch thick;

(b) heating the base and mixture deposited thereon to a temperature sufficient to drive off the volatile components of the mixture;

(c) heating the base and mixture deposited thereon to a temperature sufficient to begin the reduction of the organic metal compound in solution and to burn off the carbonaceous residue; and

(d) firing the base and the mixture deposited thereon at a temperature sufficient to melt the glass frit but not suflicient to soften the nonconductive base.

9. An electrical resistance element having a resistance per square lying in the range of 50,000-400,000 ohms, and comprising a high temperature resistant electrically nonconductive base, and a layer of glass with a noble metal in a fine state of subdivision dispersed in the glass fired onto a surface of the base, said layer being characterized 8 by the fact that its thickness is in the range of between .00005 and .00003 of an inch.

10. An electrical resistance element having a resistance per square lying in the range of 50,000400,000 ohms, and of the type comprising a film of glass in a base of high temperature resistant insulating material containing minute crystals of a noble material dispersed therein, said film having a thickness in the range of between .0002 inch and .00003 inch.

11. The resistance element of claim 10 further characterized by the fact that the film consists of superimposed substantially identical layers fused to one another, and that the thickness of each of said layers lies between on the order of .00003 and .00005 of an inch.

12. An electrical resistor composition adapted to be applied to and fired onto a ceramic base to form readily reproducible resistance elements, comprising by weight 4-9% finely ground glass frit; .04-.64% of a noble metal which is present in the composition as the metal constituent of an organic metal compound in solution of the selected noble metal; and 91-95% organic screening and viscosifying agent.

13. An electrical resistance element having a resistance per square lying in the range of 50,000-400,000 ohms, and comprising a high temperature resistant electrically nonconductive base, and a layer of glass with at least one of the metals selected from the group consisting of Au, Ag, Pt, Pd, Ir, Rh, and Ru in a fine state subdivision dispersed in the glass fired onto a surface of the base, the layer being characterized by the fact that its thickness is in the range of between .00005 and .00003 of an inch.

14. An electrical resistance element having a resistance per square lying in the range of 50,000-400,000 ohms, and of the type comprising a film of glass on a base of high temperature resistant insulating material containing minute crystals of at least one of the metals selected from the groupconsisting of Au, Ag, Pt, Pd, Ir, Rh, and Ru dispersed therein, the film having a thickness in the range of between .0002 inch and .00003 of an inch.

15. The resistance element of claim 14 further characterized by the fact that the film consists of superimposed substantially identical layers fused to one another, and that the thickness of each of said layers lies between on the order of .00003 and .00005 of an inch.

16. An electrical resistor composition adapted to be applied to and fired onto a ceramic base to form readily reproducible resistance elements, comprising by weight 4-9% finely ground glass frit; .04-.64% of at least one of the metals selected from the group consisting of Au, Ag, Pt, Pd, Ir, Rh and Ru, said metal being present in the composition as a constituent part of an organic metal compound in solution; and 91-95% organic vehicle.

17. An electrical resistance element of the type comprising a base of high temperature resistant, dielectric material having a film of glass containing minute crystals of ruthenium in excess of .5 percent by weight dispersed and bonded to the base, said ruthenium having a hexagonal close-packed crystalline structure and an acicular growth.

18. An electrical resistance element comprising a high temperature resistant, electrically nonconductive ceramic base, and a film of glass firmly attached to said base, said film containing from 05-13% by weight ruthenium, said ruthenium having a hexagonal close-packed crystalline structure and an acicular growth.

19. An electrical resistance element comprising a high temperature resist-ant, electrically nonconductive ceramic base having fired thereto a layer of resistance material comprising about 87-99.5% by weight of solidified glass and about .5-13% by weight of ruthenium in finely di vided form dispersed throughout the solidified glass in electrically conductive relationship.

20. A composition adapted to be fired onto a dielectric ceramic base to form a consistently reproducible electrical resistance path comprising an organic liquid vehicle, and

9 87-99.5% by weight of finely divided glass particles, and .5-13% by weight of finely divided ruthenium particles admixed With the organic liquid vehicle.

References Cited UNITED STATES PATENTS 2,281,843 5/1942 Jira 338-308 X 2,328,101 8/1943 Rosenblatt 117123 X 2,551,712 5/1951 Soby 1l7-123 X Raymer 338308 X DAndrea 117227 Place et a1 117227 Dumesnil 117227 X ALFRED L. LEAVI'IT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

W. L. JARVIS, Assistant Examiner. 

1. AN ELECTRICAL RESISTANCE ELEMENT OF THE TYPE COMPRISING A FILM OF GLASS ON A BASE OF HIGH TEMPERATURE RESISTANT INSULATING MATERIAL CONTAINING MINUTE CRYSTALS OF METAL DISPERSED THEREIN, CHARACTERIZED BY THE FACT THAT SAID METAL IS RUTHENIUM, THE RUTHENIUM CONTAINING GLASS FILM BEING LESS THAN .0002 INCH THICK, SAID RUTHENIUM HAVING A HEXAGONAL CLOSE-PACKED CRYSTALINE STRUCTURE AND AN ACICULAR GROWTH. 